(1) Field of the Invention
The present invention relates to sound attenuating devices which reduce the levels of noise found in air ventilation systems and more specifically air ventilation systems which employ axial flow fans of the type used in mine and industrial ventilation.
(2) Description of the Prior Art
In mine and other industrial environs air is circulated through ventilation ducts by fans, typically by axial air flow fans. Noise created in these systems can be generated from a variety of sources such as the noise and vibration of the fan itself, the noise generated by the flow of air through the fan and its associated ducting, as well as many other noise producing components found in the total ventilation system. In mine ventilation as well as in other industrial applications, the air may be laden with moisture, particulates and corrosive substances.
One prior art type of sound attenuator comprises two concentric tubular members made of metal, the inner tubular member comprising screen mesh. Sound attenuating material such as fiberglass batting or the like is positioned between the concentric tubular members. The tubular members are held in concentric relation by flanges welded at opposite ends of the tubular members. Although this sound attenuator provides for adequate sound attenuation, it has several drawbacks. During use, the sound attenuating material between the tubular members is abraided by the particulates and saturated with moisture in the air, thereby decreasing the sound attenuating properties of the material and replacement of the entire attenuator is then necessary. Moreover, because the members are made principally of metal, the corrosive substances in the air tend to corrode the metal tubular members. Furthermore, this sound attenuator tends to be quite heavy and bulky thus making it difficult to install and handle.
Examples of other prior art sound attenuators may be seen in U.S. Pat. Nos. 650,290; 2,489,048; 3,012,923; 3,507,354; and 3,718,161.